1. Field of the Invention
The present invention relates to a thin film transistor and a method for fabricating the same, more particularly, to a thin film transistor having a channel layer of which no seed exists and no grain boundary exists, or one grain boundary exists, and a method for fabricating the thin film transistor.
2. Description of Related Art
Generally, a polycrystalline silicon layer is used for various purposes as a semiconductor layer for a thin film transistor. For example, since the polycrystalline silicon layer has a high electric field effect and mobility, it can be applied to circuits operated at high speed, and it enables Complementary Metal Oxide Semiconductor (CMOS) circuit to be constructed. A thin film transistor using the polycrystalline silicon layer can also be used in an active element of an active matrix liquid crystal display (AMLCD) and a switching element and a driving element of an organic light emitting diode (OLED).
The polycrystalline silicon layer used in the thin film transistor is fabricated by direct deposition, high temperature thermal annealing or laser annealing. In case of the laser annealing, while it can be performed at low temperature, and can result in high electric field effect and mobility, many alternative technologies are being studied because of the requirement for expensive laser equipment.
At present, a method for crystallizing amorphous silicon using metal is mostly being studied since the method has merits that the amorphous silicon is promptly crystallized at a lower temperature compared with solid phase crystallization. The crystallization method using metal can be categorized into a metal induced crystallization method and a metal induced lateral crystallization method. Regardless of its category, however, the crystallization method using metal has the problem that characteristics of elements for thin film transistors are deteriorated by metal contamination.
As such a technology of forming a good polycrystalline silicon layer by controlling concentration of ions of metal through an ion injector has been developed. The technology performs high temperature annealing, rapid thermal annealing or laser irradiation and a method for crystallizing the thin film by thermal annealing after depositing a thin film by spin coating a mixture of organic film having viscous property and liquid phase metal on the polycrystalline silicon layer to flatten the surface of polycrystalline silicon layer as metal induced crystallization are developed to reduce quantity of metal and form a polycrystalline silicon layer of good quality. However, even in this case, there are problems in aspects of scale-up of grain size and uniformity of grain in the polycrystalline silicon layer.
In order to solve the foregoing problems, a method for manufacturing polycrystalline silicon layer as a crystallization method using a capping (or cover) layer has been developed as disclosed in Korean Patent Laid-open Publication No. 10-2003-0060403. The disclosed method uses an amorphous silicon layer that is formed on a substrate, and a capping layer is formed on the amorphous silicon layer. Subsequently, a seed is formed by depositing a metal catalyst layer capping layer and diffusing metal catalyst into the amorphous silicon layer through the capping layer by thermal annealing or laser annealing. A polycrystalline silicon layer is obtained using the formed seed. This method has merits in reducing metal contamination because the metal catalyst is diffused through the capping (or cover) layer.
However, the foregoing method has problems in that it is difficult to uniformly control low concentration of crystallization catalyst and control crystallization position and grain size. Particularly, the foregoing method has problems in that it is difficult to control the number of boundaries between the seeds and the grains since characteristics and uniformity of elements for thin film transistors are greatly influenced by the number of boundaries between seeds and grains formed in a channel of the thin film transistor.